Turbofan engine front section

ABSTRACT

A front section for a gas turbine engine according to an example of the present disclosure includes, among other things, a fan section including a fan hub. The fan hub includes a hub diameter supporting a plurality of fan blades including a tip diameter. A transitional entrance passage is configured to communicate flow between the fan section and a compressor section. The transitional entrance passage includes an inlet disposed at an inlet diameter and an outlet to the compressor section disposed at an outlet diameter. A method of designing a gas turbine engine is also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 14/785,872, filed on Oct. 21, 2015, which is a National Stage Entry of PCT Application No. PCT/US2014/035821, filed on Apr. 29, 2014, which claims priority to U.S. Provisional Application No. 61/821,371 filed on May 9, 2013.

BACKGROUND

A turbofan engine includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-energy exhaust gas flow. The high-energy exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines.

A speed reduction device such as an epicyclical gear assembly may be utilized to drive the fan section such that the fan section may rotate at a speed different than the turbine section so as to increase the overall propulsive efficiency of the engine. The geared architecture may be located in a front section of the engine and thereby influences how airflow paths are defined to the compressor section. Airflow efficiency into the compressor section provides increased overall engine efficiency and therefore any improvements are desirable.

Turbofan engine manufacturers continue to seek improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.

SUMMARY

A front section for a gas turbine engine according to an example of the present disclosure includes a fan section including a fan hub. The fan hub includes a hub diameter supporting a plurality of fan blades including a tip diameter. A ratio of the hub diameter to the tip diameter is between about 0.24 and about 0.36. A transitional entrance passage is configured to communicate flow between the fan section and a compressor section. The transitional entrance passage includes an inlet disposed at an inlet diameter and an outlet to the compressor section disposed at an outlet diameter. A ratio of the hub diameter to the inlet diameter is between about 0.65 and about 0.95.

In a further embodiment of any of the foregoing embodiments, the plurality of fan blades is less than twenty (20) fan blades.

In a further embodiment of any of the foregoing embodiments, the ratio of the hub diameter to the inlet diameter is between about 0.70 and about 0.90.

In a further embodiment of any of the foregoing embodiments, the fan section is configured to deliver air into a bypass duct, and a portion of air into the compressor section, with a bypass ratio defined as the volume of air delivered into the bypass duct compared to the volume of air delivered into the compressor section, and the bypass ratio being greater than about 10.

In a further embodiment of any of the foregoing embodiments, a pressure ratio across the fan section is less than about 1.5.

In a further embodiment of any of the foregoing embodiments, the compressor section includes a multi-stage low pressure compressor.

A gas turbine engine according to an example of the present disclosure includes a fan section including a fan hub. The fan hub includes a hub diameter supporting a plurality of fan blades including a tip diameter, a ratio of the hub diameter to the tip diameter being between about 0.24 and about 0.36. A compressor section includes a low pressure compressor and a high pressure compressor. A turbine section includes a fan drive turbine configured to drive the fan section and the low pressure compressor. The fan drive turbine has a greater number of stages than the low pressure compressor. The fan drive turbine has fewer stages than the high pressure compressor. A transitional entrance passage couples the fan section and the compressor section. The transitional entrance passage includes an inlet disposed at an inlet diameter and an outlet disposed at an outlet diameter. The inlet is adjacent to the fan section. The outlet is adjacent to the compressor section. A ratio of the hub diameter to the inlet diameter is between about 0.65 and about 0.95.

In a further embodiment of any of the foregoing embodiments, the low pressure compressor is a multi-stage compressor.

In a further embodiment of any of the foregoing embodiments, the ratio of the hub diameter to the inlet diameter is between about 0.70 and about 0.90.

In a further embodiment of any of the foregoing embodiments, the fan section is configured to deliver air into a bypass duct, and a portion of air into the compressor section, with a bypass ratio defined as the volume of air delivered into the bypass duct compared to the volume of air delivered into the compressor section. The bypass ratio is greater than about 6.

A further embodiment of any of the foregoing embodiments includes a geared architecture configured to rotate the fan hub at a lower relative speed than the fan drive turbine.

In a further embodiment of any of the foregoing embodiments, the plurality of fan blades is less than twenty (20) fan blades.

In a further embodiment of any of the foregoing embodiments, the low pressure compressor is a three stage compressor.

In a further embodiment of any of the foregoing embodiments, the ratio of the hub diameter to the inlet diameter is between about 0.70 and about 0.90.

In a further embodiment of any of the foregoing embodiments, the turbine section includes a high pressure turbine configured to drive the high pressure compressor, the high pressure turbine including at least two stages.

A method of designing a gas turbine engine according to an example of the present disclosure includes providing a fan section including a fan hub. The fan hub includes a hub diameter supporting fewer than 26 fan blades to define a tip diameter. A ratio of the hub diameter to the tip diameter is between about 0.24 and about 0.36. The method includes providing a compressor section coupled to a turbine section. The turbine section is configured to drive the fan section providing a transitional entrance passage coupling the fan section and the compressor section. The transitional entrance passage includes an inlet disposed at an inlet diameter and an outlet to the compressor section disposed at an outlet diameter. A ratio of the hub diameter to the inlet diameter is between about 0.70 and about 0.90.

In a further embodiment of any of the foregoing embodiments, the compressor section includes a low pressure compressor and a high pressure compressor. The low pressure compressor is a multi-stage compressor, and the high pressure compressor has a greater number of stages than the low pressure compressor.

In a further embodiment of any of the foregoing embodiments, the hub diameter supports fewer than 20 fan blades.

In a further embodiment of any of the foregoing embodiments, the fan section is configured to deliver air into a bypass duct, and a portion of air into the compressor section, with a bypass ratio defined as the volume of air delivered into the bypass duct compared to the volume of air delivered into the compressor section. The bypass ratio is greater than about 10.

A further embodiment of any of the foregoing embodiments includes providing a geared architecture configured to rotate the fan hub at a lower relative speed than the turbine section.

Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples.

These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example turbofan engine.

FIG. 2 is a schematic view of an example front section of a turbofan engine.

FIG. 3 is a schematic view of another example front section of a turbofan engine.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an example turbofan engine 20 that includes a fan section 22 and a core engine section 18 that includes a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmenter section (not shown) among other systems or features. The fan section 22 drives air along a bypass flow path B while the compressor section 24 draws air in along a core flow path C where air is compressed and communicated to a combustor section 26. In the combustor section 26, air is mixed with fuel and ignited to generate a high pressure exhaust gas stream that expands through the turbine section 28 where energy is extracted and utilized to drive the fan section 22 and the compressor section 24.

Although the disclosed non-limiting embodiment depicts a turbofan gas turbine engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines; for example a turbine engine including a three-spool architecture in which three spools concentrically rotate about a common axis and where a low spool enables a low pressure turbine to drive a fan via a gearbox, an intermediate spool that enables an intermediate pressure turbine to drive a first compressor of the compressor section, and a high spool that enables a high pressure turbine to drive a high pressure compressor of the compressor section.

The example engine 20 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 that connects a fan 42 and a low pressure (or first) compressor section 44 to a low pressure (or first) turbine section 46. The inner shaft 40 drives the fan 42 through a speed change device, such as a geared architecture 48, to drive the fan 42 at a lower speed than the low speed spool 30. The high-speed spool 32 includes an outer shaft 50 that interconnects a high pressure (or second) compressor section 52 and a high pressure (or second) turbine section 54. The inner shaft 40 and the outer shaft 50 are concentric and rotate via the bearing systems 38 about the engine central longitudinal axis A.

A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. In one example, the high pressure turbine 54 includes at least two stages to provide a double stage high pressure turbine 54. In another example, the high pressure turbine 54 includes only a single stage. As used herein, a “high pressure” compressor or turbine experiences a higher pressure than a corresponding “low pressure” compressor or turbine.

The example low pressure turbine 46 has a pressure ratio that is greater than about 5. The pressure ratio of the example low pressure turbine 46 is measured prior to an inlet of the low pressure turbine 46 as related to the pressure measured at the outlet of the low pressure turbine 46 prior to an exhaust nozzle.

A mid-turbine frame 58 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 58 further supports bearing systems 38 in the turbine section 28 as well as setting airflow entering the low pressure turbine 46.

Airflow through the core airflow path C is compressed by the low pressure compressor 44 then by the high pressure compressor 52 mixed with fuel and ignited in the combustor 56 to produce high speed exhaust gases that are then expanded through the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 58 includes vanes 60, which are in the core airflow path and function as an inlet guide vane for the low pressure turbine 46. Utilizing the vane 60 of the mid-turbine frame 58 as the inlet guide vane for low pressure turbine 46 decreases the length of the low pressure turbine 46 without increasing the axial length of the mid-turbine frame 58. Reducing or eliminating the number of vanes in the low pressure turbine 46 shortens the axial length of the turbine section 28. Thus, the compactness of the turbofan engine 20 is increased and a higher power density may be achieved.

The disclosed turbofan engine 20 in one example is a high-bypass geared aircraft engine. In a further example, the turbofan engine 20 includes a bypass ratio greater than about six (6), with an example embodiment being greater than about ten (10). The example geared architecture 48 is an epicyclical gear train, such as a planetary gear system, star gear system or other known gear system, with a gear reduction ratio of greater than about 2.3.

In one disclosed embodiment, the turbofan engine 20 includes a bypass ratio greater than about ten (10:1) and the fan diameter is significantly larger than an outer diameter of the low pressure compressor 44. It should be understood, however, that the above parameters are only exemplary of one embodiment of a gas turbine engine including a geared architecture and that the present disclosure is applicable to other gas turbine engines.

A significant amount of thrust is provided by airflow through the bypass flow path B due to the high bypass ratio. The fan section 22 of the engine 20 is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft., with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of pound-mass (lbm) of fuel per hour being burned divided by pound-force (lbf) of thrust the engine produces at that minimum point.

“Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.50. In another non-limiting embodiment the low fan pressure ratio is less than about 1.45.

“Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram °R)/(518.7°R)]^(0.5). The “Low corrected fan tip speed”, as disclosed herein according to one non-limiting embodiment, is less than about 1150 ft/second.

The example gas turbine engine includes the fan 42 that comprises in one non-limiting embodiment less than about twenty-six (26) fan blades. In another non-limiting embodiment, the fan section 22 includes less than about twenty (20) fan blades. Moreover, in one disclosed embodiment the low pressure turbine 46 includes no more than about six (6) turbine rotors schematically indicated at 34. In another non-limiting example embodiment the low pressure turbine 46 includes about three (3) turbine rotors. A ratio between the number of fan blades 42 and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine 46 provides the driving power to rotate the fan section 22 and therefore the relationship between the number of turbine rotors 34 in the low pressure turbine 46 and the number of blades 42 in the fan section 22 disclose an example turbofan engine 20 with increased power transfer efficiency.

The gas turbofan engine 20 includes a front section 62 extending from the fan section 22 axially aft to bearing assembly 108 supporting a forward portion of the low speed spool 30. The front section 62 includes the fan section 22, the geared architecture 48, and compressor inlet passage 88 part of the core flow path C. The fan section 22 includes a fan hub 64 that supports the fan blades 42.

The fan hub 64 supports each of the blades 42 for rotation about the axis A. Each of the blades 42 includes a blade tip 68. A tip diameter 70 is disposed between opposing blade tips 68 and defines the diameter of the fan section 22. The blades 42 extend from a root portion 45 supported within a fan hub 64. The fan hub 64 defines a hub diameter 66. The hub diameter 66 is related to the tip diameter 70 according to a ratio that reflects a size of the bypass flow path B related to the core engine section 18. In the disclosed embodiment the ratio of hub diameter 66 to the tip diameter 70 is between about 0.24 and about 0.36.

The core flow path C includes a compressor inlet passage 88 that is disposed annularly about the geared architecture 48. The compressor inlet passage 88 includes an inlet 90 into supporting structure for the geared architecture 48 and the fan hub 64 and an outlet 94 aft of the supporting structure. The outlet 94 directs air into a first stage of the low pressure compressor 44. The hub 64, inlet 90 and outlet 94 define a path for air entering the turbofan engine 20 and entering the low pressure compressor 44.

Referring to FIG. 2, with continued reference to FIG. 1, the inlet 90 is set at an inlet diameter 92 and the outlet 94 is set at an outlet diameter 96. The inlet 90 is at the leading edge and innermost radius of the first vane 102 aft of the fan section 22. The outlet 94 is the innermost radius of the first rotating airfoil 120 of the first or low pressure compressor 44. The hub diameter 66, inlet diameter 92 and outlet diameter 96 define a transitional flow path 106 into the low pressure compressor 44. The transitional flow path 106 includes complex flow fields with swirl components that are turned by vanes 102 and 104 within the compressor inlet passage 88 to provide desired axially oriented flows.

The transitional flow path 106 includes relatively low diametrical changes to provide a smooth aerodynamic flow path into the low pressure compressor 44. The shape of the transitional flow path 106 greatly improves and simplifies aerodynamic flow performance through the compressor inlet passage 88 to increase engine efficiency.

The shape of the transitional flow path 106 is defined by ratios between the hub diameter 66, inlet diameter 92 and outlet diameter 96. The disclosed transitional flow path 106 includes a ratio of the inlet diameter 92 to the outlet diameter 96 that is between about 1.10 and about 1.64, or more narrowly between about 1.24 and about 1.44. The transitional flow path 106 further includes a ratio of the hub diameter 66 to the inlet diameter 92 that is between about 0.65 and about 0.95, or more narrowly between about 0.70 and about 0.90.

In one example engine embodiment, the hub diameter 66 is between about 21.0 inches (53.34 cm) and about 26.0 inches (66.04 cm). The inlet diameter 92 is between about 26.5 inches (67.31 cm) and about 33.0 inches (83.82 cm). The outlet diameter 96 is between about 20.0 inches (50.8 cm) and about 24.5 inches (62.23 cm).

The inlet diameter 92 is the largest of the diameters 66, 92 and 96 defining the transitional flow path 106 and defines a necessary inflection point from the convergence of the root portion 45 of the fan blade 42 that provides desired aerodynamic performance.

The transitional flow path 106 between the hub diameter 66, the inlet diameter 92 and outlet diameter 96 is enabled by a gear diameter 98 defined by the geared architecture 48 and by the axial and radial position of the forward bearing assembly 108. The forward bearing assembly 108 is positioned axially and radially relative to the low pressure compressor 44 to enable the subtle changes in the transitional flow path 106. Accordingly, the inlet diameter 92, and therefore the desired inflection point is enabled by the size of the geared architecture 48, and the outlet diameter 96 is enabled by the size and position of the forward bearing assembly 108.

The geared architecture 48 includes a sun gear 76 driven by the low pressure turbine 46 through the inner shaft 40. The sun gear 76 drives intermediate planetary gears (either planet gears or star gears) 78 supported for rotation by journal bearings 100. A carrier 82 supports the journal bearings 100 and thereby the planetary gears 78. A ring gear 80 circumscribes the planetary gears 78. In this example, the ring gear 80 is attached to drive the fan hub 64 about the axis A. The carrier 82 is fixed and holds the intermediate planetary gears 78 from rotation about the axis A.

The geared architecture 48 illustrated in FIG. 1 is a star epicyclical configuration where the ring gear 80 is driven about the axis A and the carrier 80 is fixed to a portion of the engine static structure 36. However, other gear configurations are within the contemplation of this disclosure.

Referring to FIG. 3, another geared architecture 49 is shown that includes a sun gear 75 that drives planet gears 79 supported in a carrier 83 that is attached to drive a fan hub 65. A ring gear 81 circumscribes the planet gears 79 and is fixed to a portion of the engine static structure 36. The geared architecture 49 drives the fan hub 65 through rotation of the planet gears 79 and carrier 83 about the axis A and is referred to as a planet epicyclical gear configuration. The disclosed features and size are applicable to either of the disclosed geared architectures 48, 49 illustrated schematically in FIGS. 2 and 3. Further explanation and disclosure are explained with regard to the geared architecture 48 illustrated in FIG. 2, but is just as applicable to the embodiment illustrated and explained in FIG. 3.

Referring back to FIGS. 1 and 2, the carrier 82 includes an outer periphery 85 and the ring gear 80 includes the gear diameter 98 that combines to define a gear volume 72. The gear diameter 98 defined by the ring gear 80 and carrier 82 define the boundary of the gear volume 72. The gear volume 72 includes the elements of the geared architecture such as the journal bearings, 100, carrier 80, sun gear 76, planetary gears 78 and ring gear 80. The gear volume 72 does not encompass the mounting and flexible structures that may be utilized to support the geared architecture.

The gear volume 72 is the annular volume about the axis A, defined within the bounds of the gear diameter 98 and axial length 74. The axial length 74 of the geared architecture 48 includes the carrier 82. In the disclosed example, the geared architecture 48 includes an axial length 74 between about 4.16 inches (10.56 cm) and about 6.90 inches (17.53 cm).

The geared architecture 48 provides for the transmission of input horsepower 84 from the low pressure turbine 46 to output horsepower 86 to the fan section 22. The efficient transmission of input horsepower 84 is enabled by the configuration defining the gear volume 72. In this example, the gear volume is between about 1318 in³ (21,598 cm³) and about 1977 in³ (32,397 cm³).

The gearbox volume is necessary for the transfer of power from the fan drive or low pressure turbine 46 to the fan section 22. The gear diameter 98 is held close to the fan hub diameter 66 to define the flowpath 106 to be as short and unvarying in diameter as possible. The short and unvarying diameter of the transitional flow path 106 enables preferred pressure recovery between the fan blade root 45, the inlet 92 and the outlet 94 to the first rotating stage of the first or low pressure compressor 44.

In one example, the range of gear volume 72 is provided for an engine 20 that generates thrust ranging between about 21,000 lbf (93,412 N) and 35,300 lbf (157,022 N). The thrust generated is a function of the efficiency of the engine configuration and of the transfer of horsepower through the geared architecture 48. A measure of the efficiency of the geared architecture for a give volume is a power transfer parameter (PTP) and is defined as the power transferred through the geared architecture 48 divided by the gear volume 72 and multiplied by an overall gear ratio, a set out in Equation 1. Power Transfer Parameter=[Power Transferred (HP)/Gear Volume (in³)]×overall gear ratio.  Equation 1:

The PTP provides a normalized factor for comparison between geared architectures for different engine configurations. Moreover the gear ratio accounts for the extra work performed for higher gear ratios. Embodiments of the geared turbofan engine that include the disclosed geared architecture 48 gear volumes 72 and that generate thrust ranging between about 21,000 lbf (93,412 N) and 35,300 lbf (157,022 N) include a PTP of between about 430 and about 645.

The PTP of the example geared architecture 48 enables increased transfer of power while maintaining a size and volume that further enables the transitional flow path 106 orientations that provide desired aerodynamic flow properties.

The forward bearing assembly 108 is disposed at an axial distance 110 from the outlet 94 to support rotation of the forward portion of the low speed spool 30 including the low pressure compressor 44. The position of the forward bearing assembly 108 provides a desired balance and rotational support of the low pressure compressor 44. Placement of the forward bearing assembly 108 is desired within a mid-region of the compressor 44 and requires a radial space sufficient to support lubricant and cooling features required during operation. Accordingly, a diameter 112 of the bearing assembly 108 has a direct effect on the configuration of the low pressure compressor 44 and thereby the position of the outlet 94. Moreover, the axial distance 118 from the forward tip of the hub 64 to the bearing assembly 108 is enabled by the size and volume of the geared architecture 48 and combined with the position of the forward bearing assembly 108 enables the desirable design of the transitional flow path 106.

In one disclosed dimensional engine embodiment the diameter 112 measured to a center point of the bearing assembly 108 is between about 7.25 inches (18.4 cm) and about 9.0 inches (22.86 cm). The axial distance 110 is between about 4.25 inches (10.79 cm) and 9.00 inches (22.86 cm). An overall axial length 118 of the front section 62 from the hub diameter 66 at the forward portion of the fan hub 64 to the forward bearing assembly is between about 33.00 inches (83.82 cm) and about 67.50 inches (171.45 cm). The axial distance 110 between the outlet 94 and the bearing assembly 108 enable the desired reduced length of the forward section 62.

The disclosed dimensional embodiment is only one example enabling the disclosed configuration of the transitional flow path 106. The configuration of disclosed engine embodiments is reflected in a ratio of the overall length 118 to the axial distance 110 that is between about 3.6 and 15.8. Moreover, a ratio between the outlet diameter 96 and the bearing assembly diameter 112 is between about 2.25 and 3.35. These ratios reflect the configuration that enables the radial and axial position of the outlet 94.

The axial length 74 of the geared architecture 48 further enables the desired relatively flat transitional flow path 106. The volume of the geared architecture 48 enables the power transfer to the fan hub 64 and is a factor determined by the axial length 74 and the gear diameter 98. Decreasing the gear diameter 98 enables a corresponding reduction in axial length 74 that in turn enables the desired configuration of the transitional flow path 106.

Therefore, a relationship between the axial length 74 of the geared architecture and the overall length 118 of the front section 62 further reflects the disclosed configuration of the transitional flow path 106 and engine 20. A ratio of the overall length 118 as related to the axial length 74 of the geared architecture 48 is between about 4.8 and about 16.2. The ratio of the overall length 118 to the axial length 74 reflects the disclosed geared architecture 48 including the gear diameter 98 and volume 72 that the desired configuration of the transitional flow path 106 and front section 62.

Accordingly, the gear volume 72, gear diameter 98, and axial length 74 of the geared architecture along with the location of the forward bearing assembly 108 enable an efficient transitional flow path 106 in the disclosed compact front section 62.

Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure. 

What is claimed is:
 1. A front section for a gas turbine engine comprising: a fan section including a fan hub, the fan hub including a hub diameter supporting a plurality of fan blades including a tip diameter, a ratio of the hub diameter to the tip diameter being between 0.24 and 0.36; and a transitional entrance passage configured to communicate flow between the fan section and a compressor section, the transitional entrance passage including an inlet disposed at an inlet diameter and an outlet disposed at an outlet diameter, the outlet adjacent to the compressor section, and a ratio of the hub diameter to the inlet diameter being between 0.70 and 0.90; and wherein the outlet is axially forward of a forwardmost stage of the compressor section relative to an engine longitudinal axis, and a ratio of the inlet diameter to the outlet diameter is between 1.10 and 1.64.
 2. The front section as recited in claim 1, wherein the plurality of fan blades is less than twenty (20) fan blades.
 3. The front section as recited in claim 1, wherein a pressure ratio across the fan section is less than 1.5 across the the plurality of fan blades.
 4. The front section as recited in claim 1, wherein the compressor section includes a multi-stage low pressure compressor.
 5. A gas turbine engine comprising: a fan section including a fan hub, the fan hub including a hub diameter supporting a plurality of fan blades including a tip diameter, a ratio of the hub diameter to the tip diameter being between 0.24 and 0.36, and an outer housing surrounding the fan blades to establish a bypass duct; a compressor section including a low pressure compressor and a high pressure compressor; a turbine section including a fan drive turbine configured to drive the fan section and the low pressure compressor, the fan drive turbine having a greater number of stages than the low pressure compressor, and the fan drive turbine having fewer stages than the high pressure compressor; and a transitional entrance passage coupling the fan section and the compressor section, the transitional entrance passage including an inlet disposed at an inlet diameter and an outlet disposed at an outlet diameter, the inlet adjacent to the fan section, the outlet adjacent to the compressor section, and a ratio of the hub diameter to the inlet diameter being between 0.70 and 0.90; and wherein the outlet is axially forward of a forwardmost stage of the compressor section relative to an engine longitudinal axis, and a ratio of the inlet diameter to the outlet diameter is between 1.10 and 1.64.
 6. The gas turbine engine as recited in claim 5, wherein the low pressure compressor is a multi-stage compressor.
 7. The gas turbine engine as recited in claim 6, comprising a geared architecture configured to rotate the fan hub at a lower relative speed than the fan drive turbine.
 8. The gas turbine engine as recited in claim 5, wherein the plurality of fan blades is less than twenty (20) fan blades.
 9. The gas turbine engine as recited in claim 8, wherein the low pressure compressor is a three stage compressor.
 10. The gas turbine engine as recited in claim 9, wherein the turbine section includes a high pressure turbine configured to drive the high pressure compressor, the high pressure turbine including at least two stages.
 11. The gas turbine engine as recited in claim 6, wherein the fan section includes an outer housing that surrounds the plurality of fan blades, the turbine section includes a high pressure turbine configured to drive the high pressure compressor, the high pressure turbine including at least two stages, the fan drive turbine having a greater number of stages than the high pressure turbine, the low pressure compressor includes three compressor stages, the high pressure compressor having a greater number of stages than the low pressure compressor, the plurality of fan blades is less than twenty (20) fan blades, and a ratio between the number of fan blades and the number of turbine rotors of the fan drive turbine is between 3.3 and 8.6.
 12. The gas turbine engine as recited in claim 11, wherein the fan drive turbine includes three turbine rotors.
 13. The gas turbine engine as recited in claim 12, wherein a pressure ratio across the fan section is less than 1.45 across the the plurality of fan blades.
 14. The gas turbine engine as recited in claim 13, comprising a geared architecture including an epicyclic gear train configured to rotate the fan hub at a lower relative speed than the fan drive turbine.
 15. The gas turbine engine as recited in claim 14, wherein the geared architecture is a planetary gear system.
 16. The gas turbine engine as recited in claim 11, wherein the fan drive turbine includes five turbine rotors, a pressure ratio across the fan section is less than 1.45 across the plurality of fan blades, and further comprising: a geared architecture including an epicyclic gear train configured to rotate the fan hub at a lower relative speed than the fan drive turbine, wherein the geared architecture is a planetary gear system. 